1. Field of the Invention
The present invention relates to a laminated structure having excellent transparency and gas and vapor impermeability and which is particularly suitable for use as a material for forming containers or as a packaging film or sheet for food products, beverages and pharmaceutical products.
2. Description of the Prior Art
Flexible or rigid packaging materials for packaging food products, beverages or pharmaceutical products are required to meet strict requirements for physical properties to safeguard human health. More specifically, such a packaging material should provide a barrier to gases, water vapor and ultraviolet rays. Further, depending on the intended end use, resistance to cold and/or hot water is desirable. Various laminated structures have been proposed to meet such requirements.
For example, polyester film, particularly, polyethylene terephthalate film is an excellent food packaging material and is used widely in various fields. However, a polyethylene terephthalate (PET) film is an unsatisfactory gas barrier. In order to cover this disadvantage, a polyethylene terephthalate film is conventionally coated with a polyvinylidene chloride or laminated to an aluminum foil, a polyvinyl alcohol film or a saponified ethylene-vinyl acetate copolymer film.
However, an aluminum foil conceals the contents of the package and spoils the attractiveness of the goods, while coating a polyethylene terephthalate film with a polyvinylidene chloride yields a coating with unacceptably low bond strength. Accordingly, laminated films of a polyethylene terephthalate layer and a saponified ethylene-vinyl acetate copolymer layer have become widely used in recent years. Since a polyethylene terephthalate film and a saponified ethylene-vinyl acetate copolymer film will not adhere directly to each other upon coextrusion, they are conventionally laminated using an adhesive. However, polyurethane adhesives, which are commonly used as adhesives for laminating a polyethylene terephthalate film and a saponified ethylene-vinyl acetate copolymer film, present certain problems including the requirement for aging after processing and the possibility that compounds of low molecular weight might migrate into the packaged foodstuff.
To obviate such problems, one method for forming such a laminated film without an adhesive has been proposed. According to the proposed method, a surface of a thermoplastic resin film is treated by a corona discharge, then another thermoplastic resin film is laminated to the treated surface of the former film by hot-pressing, and then the laminated structure is stretched. However, the interlayer adhesive strength of such a laminated film is as low as 200 g/15 mm, and hence delamination is liable to occur. Furthermore, as a practical matter, the two or more component layers of the laminate must have an overlapping stretchable temperature range, and hence only a limited number of film combinations are possible. Therefore, a laminated film produced by such a process is unsuitable for uses in which it would be subjected to hot-forming or heat-sterilization, and accordingly, there are many restrictions on the practical applications of such a conventional laminated film.